The invention pertains to an emergency stop circuit for a direct current elevator drive, the drive including a direct current motor with an armature winding and a field winding, where in the case of an emergency stop a damping resistance is switched across the armature winding and current is shunted to the field winding to produce a torque for the deceleration of the elevator car and a counterweight.
In elevator systems, essential system defects, such as, for example, power outage, defects in the security circuit etc., cause an emergency stop, where the elevator car is immediately braked or, stopped. In the case of a direct current drive, the motor is disconnected from the electrical supply and a resistance switched across the armature circuit of the motor. The voltage generated across the resistance supplies the field winding of the motor with current. The armature and the field together generate a motor torque for the deceleration of the elevator car and the counterweight. Jointly the deceleration of the elevator drive is composed of the motor torque and the application of braking torque by a mechanical brake. The total torque has to be calculated in such a way that the braking of the elevator car at full load is assured within a certain stopping distance.
It is a disadvantage of the known emergency stop devices that for a deceleration designed for full load, the elevator car is excessively braked, for example at partial load, such as with a single passenger. In this case, undesired forces act on the passenger and on the parts of the elevator. Besides, there exists also the danger that the counterweight can jump in an upward direction transmitting a shock to the car.